A typical receiver for a radio frequency signal (RF signal) comprises a combination of an amplifier and a mixer for signal amplification and frequency conversion. The amplifier, usually a low-noise amplifier (LNA), receives the RF signal, amplifies the RF signal and feeds the amplified RF signal to the mixer which in addition receives a local signal from a local oscillator (LO). The local oscillator signal has a frequency which is different from the frequency of the RF signal. The mixer, which is a nonlinear device, generates an output signal that includes more frequencies than the frequencies of the RF signal and the local signal. The output signal is usually filtered to block undesired frequencies which include the original frequencies, their harmonics and their sum frequencies.
The amplifier and mixer used in such a receiver should ideally exhibit several desired parameters and characteristics which are usually used to describe the performance of the amplifier and the mixer. For example, an amplifier should exhibit a high power gain, a low noise figure and the capability of handling large input signals without intermodulation distortion (IMD). The power gain is defined as the ratio of the signal power at the output port to the signal power at the input port. The noise figure is defined as the ratio of the signal-to-noise ratio (SNR) at the input port to the signal-to-noise ratio at the output port.
The intermodulation distortion refers to undesired frequency components which are caused when a signal having two or more sinusoidal frequencies f.sub.1, f.sub.2 is applied to a nonlinear amplifier. The output signal of such an amplifier contains the additional undesired frequency components called intermodulation products. The output signal will contain, for example, frequency components at DC, f.sub.1, f.sub.2, 2f.sub.1, 2f.sub.2, 3f.sub.1, 3f.sub.2, f.sub.1 +/-f.sub.2, 2f.sub.1 +/-f.sub.2 and 2f.sub.2 +/-f.sub.1. The frequencies 2f.sub.1 and 2f.sub.2 are the second harmonics, 3f.sub.1, and 3f.sub.2 are the third harmonics, f.sub.1 +/-f.sub.2 are the second-order intermodulation products (the sum is 2), and 2f.sub.1 +/-f.sub.2 and 2f.sub.2 +/-f.sub.1 are the third-order intermodulation products (the sum is 3). The third-order intermodulation products are close to the fundamental frequencies f.sub.1 and f.sub.2 and fall within a bandwidth in which the amplifier amplifies, producing distortion in the output signal.
A parameter to evaluate the third-order intermodulation products is the so-called third-order intercept point (IP3). This point is defined by means of a graphical analysis using the output power of the third-order intermodulation product as a function of the input power and the output power of the fundamental component at fl as a function of the input power. The intercept point is defined as the point at which the two (linearized) graphs intercept. The higher the intercept point, the better the suppression of the third-order intermodulation product and the less disturbed is the output signal of the amplifier.
The mixer should exhibit, inter alia, a high conversion gain, a low noise figure and also a high third-order intercept point (IP3). The conversion gain is defined as the ratio of the output power of the IF signal to the input power of the RF signal.
The combination of the LNA amplifier and the mixer is configurable to operate in receivers which are adapted for various applications. For example, the combination can be used in TV receivers or in phones for a radio communications system. One example of a radio communications systems is a cellular system that is in accordance with a particular standard, such as "Global System for Mobile Communications" (GSM), "Advanced Mobile Phone System" (AMPS) or "Code Division Multiple Access" (CDMA).
These standards have different requirements and specifications for the combination of the LNA amplifier and the mixer, for example, with respect to linearity, noise figure and intermodulation distortions. Particularly the CDMA standard requires that the LNA amplifiers and mixers in CDMA phones have simultaneously both a high IP3 and a low noise figure.
To achieve this combination of difficult specifications, current solutions implement circuit architectures, including RF transistors (SFET, HBT), in gallium arsenide (GaAs) technology. Processes to manufacture the circuits and the RF transistors in GaAs technology, however, are expensive compared to processes in silicon technology.